The present invention relates to printing systems. More specifically, the present invention relates to a method of using spatially controlled cooling profiles to maintain uniform temperature of ink across the active zone of an acoustic ink printhead.
As computing products continue to drop in price while increasing in power, printing technology is driven by the need to reduce prices while improving printer resolution. One technology under development is acoustic ink printing (AIP). AIP focuses acoustic energy to eject droplets of a fluid from a free surface onto a recording medium. The fluid is typically ink, although in specialized applications, the fluid may be a molten solder, a hot melt wax, a color filter material, a resist, and various other chemical and biological compounds.
In AIP applications, a print head includes droplet sources that eject and deposit droplets on a receiving medium in a predetermined, controlled fashion. Each droplet sources includes a well containing ink and a transducer that agitates the ink and causes the ejection of droplets of ink from the well. A variety of manufacturing techniques, such as semiconductor processing techniques, may be used to form the transducer, the well, and the circuitry driving the transducer.
FIG. 9 illustrates a cross sectional view of a typical droplet source 90 shortly after ejection of a droplet 104 of marking fluid 108 and before a mound 112 on a free surface 116 of marking fluid 108 has relaxed. A radio frequency (RF) source 120 provides a RF drive energy of around 100 to 200 Megahertz (MHz) to a driver element such as a transducer 124 via bottom electrode 128 and top electrode 132. In one embodiment, the transducer is a piezoelectric transducer. The acoustic energy from the transducer passes through a base 136 into an acoustic lens 140. Acoustic lens 140 is often a Fresnel lens that focuses the received acoustic energy into a focused acoustic beam 138 which terminates in a small focal area near free surface 116. When sufficient acoustic energy is properly focused on free surface 116, a mound 112 is formed and a droplet 104 is ejected. A detailed description of a droplet source or xe2x80x9cdroplet ejectorxe2x80x9d is provided in U.S. Pat. No. 5,565,113 by Hadimioglu et al. entitled xe2x80x9cLithographically Defined Ejection Unitsxe2x80x9d issued Oct. 15, 1996 and hereby incorporated by reference.
A typical print head, such as an AIP print head, includes arrays of droplet sources. Tight control of the droplet size and droplet velocity at each droplet source is important to obtain a high resolution accurate image. Variations in droplet size and/or velocity from droplet sources on the same printhead reduce the accuracy and uniformity of images created by the AIP system. Thus such variations should be minimized.
In order for an acoustic ink printer (AIP) to produce a high quality image, each droplet source on the AIP printhead should be designed to output droplets of uniform size and velocity. It has been found that as ink flows across the printhead from an ink supply or xe2x80x9csourcexe2x80x9d to an ink outlet or xe2x80x9cdrainxe2x80x9d, the ink absorbs power from the many transducers distributed across a printhead. The absorbed power heats up the ink to produce an uneven temperature distribution in the ink. Uneven ink temperatures result in the output of nonuniform droplet sizes and velocities. In particular, warmer ink at droplet sources near the ink outlet results in the output of larger and higher velocity droplets compared to droplets output by droplet sources located across the printhead near the ink source. The warmer ink near the ink outlet results from energy transducer energy, both acoustic and thermal absorbed by the ink as it flows from the ink supply to the outlet. AIP printheads which are heated in order to eject phase change inks are particularly susceptible to these effects due the their relatively high viscosity (4-20 cp) resulting in high power dissipation compared to aqueous inks. The nonuniform droplet sizes and velocities degrade image quality.
In order to generate uniform droplet sizes and velocities from different droplet sources distributed across an AIP print head, a system to maintain the uniformity of ink temperature across the printhead is described. In one embodiment of the invention, a heat absorbing medium is placed on the opposite side of a substrate from the ink flowing across the printhead. The cooling effectiveness of the heat absorbing medium and the distance from the heat absorbing medium to the ink is adjusted to almost exactly compensate for the heating of the ink as it flows across the printhead such that the temperature difference in ink distributed across the print head is minimized. Alternative embodiments of the invention may adjust the flow of a cooling fluid across the backside of a printhead. As used in the following description, the backside of the printhead is a surface of the printhead upon which transducers are mounted. The heat characteristics and the flow of the cooling fluid are adjusted such that the ink temperature stays constant as the ink flows across the printhead.